The MPEG-1 standard was developed in response to the industry need of implementing an efficient way of storing and retrieving video information on storage media of the digital type, as, for example, CD-ROMs. Of course, the MPEG-1 standard is also a powerful tool for efficiently storing data on storage media, such as DATs, Winchester disks, optical disks and ISDN and LAN networks. A more efficient version of the standard, called MPEG-2 has been developed in support of bitrate requisites in the field of digital video transmission applications. The standard has been generally accepted for digital TV systems, for compressing TV-resolution pictures, entirely interlaced, up to a bitrate of about 20 Mbps. A special version of the MPEG-2 standard is expected to be used in future generation HDTV systems.
The MPEG standard incorporates and utilizes important algorithms and criteria defined by previous international standards, such as, for example, the CCITT motion vectors determination algorithm H.261 and the ISO 10918 standard of the ISO JPEG Committee for the coding of still pictures. A definition of the MPEG standard (1 and 2), as well as an exhaustive description of the different techniques of implementation and the relative coding and decoding systems of the data pertaining to compressed video pictures according to the MPEG standards are described in a wealth of articles and publications on the subject, among which the following can be mentioned:
Draft International ISO/IEC DIS 13818-2 "Information technology--Generic coding of moving pictures and associated audio information". PA1 "MPEG coding and transport system" by Leonardo Chiariglione, Digital Television Broadcasting--Proceedings. PA1 "The MPEG video compression algorithm" by Didier J. Le Gall, Signal Processing Image Communication, Elsevier Science Publishers B.V., Vol. 4, No. 2, April 1992. PA1 Digest No. 1995/012, Electronics Division, Institution of Electrical Engineers--London, Colloquium on: "MPEG-2--what it is and what it isn'2". PA1 "An Overview of the MPEG Compression Algorithm" Technical Note released by SGS-THOMSON MICROELECTRONICS (An 529/0294). PA1 Datasheet "STi3500A" Datasheet of SGS-THOMSON MICROELECTRONICS. PA1 "STi3520A--Advanced Information for an MPEG Audio/MPEG-2 Video Integrated Decoder" (June 1995). PA1 A "Bit buffer", that is, a buffer for compressed data that the MPEG-2 standard fixes at 1.75Mbits plus an extra amount, for example of 834.722 bits, in consideration of a nonideal process of decompression being actually implemented. PA1 A first "I-frame buffer" for the decompressed Intra-Picture or briefly I-picture, in a 4:2:0 format. PA1 A second "P-frame buffer" for the decompressed Predicted-Picture or briefly P-picture, in a 4:2:0 format. PA1 A third "B-frame buffer" for the decompressed Bidirectionally Predicted Picture or briefly B-picture, in a 4:2:0 format, eventually optimized so to require a reduced amount of memory, that is of 0.7407 or 0.6111 of a frame respectively in the case of a PAL or NTSC system.
According to a typical architecture of an MPEG-2 decoder, such as that shown in FIG. 3 of the publication No. STi3520A relative to an MPEG Audio/MPEG-2 Video integrated decoder marketed by SGS-THOMSON MICROELECTRONICS, herein reproduced as FIG. 1, there exist well defined requisites of video memory. The capacity of an external DRAM memory that, for a PAL and NTSC application, capable of supporting 16 Mbits PAL video signals, can be estimated as follows. Considering that both the MPEG-2 video decoder and the MPEG audio decoder access a unique external DRAM memory of 16Mbits, through a common interface. The audio decoder may require access to only 131.072 bits leaving the remaining 16.646.144 bits available for satisfying the requisites of the MPEG-2 video decoder. The video memory, according to a customary scheme, can be configured as follows.
According to the known MPEG-2 standard technique, and regardless of dealing with an I, P or B-picture, dependingly on the type of video standard, each "frame buffer" in the format 4:2:0 occupies an amount of memory given by the following table.
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720 .times. 576 .times. 8 for the luma 3.317.760 = 4.976.640 (luminance) (Y) bits bits 360 .times. 288 .times. 8 for the U chroma 829.440 (chrominance U) bits 360 .times. 288 .times. 8 for the V chroma 829.440 (chrominance V) bits NTSC 720 .times. 480 .times. 8 for the luma 2.764.800 = 4.147.200 (luminance) (Y) bits bits 360 .times. 240 .times. 8 for the U chroma 691.200 (chrominance U) bits 360 .times. 240 .times. 8 for the V chroma 691.200 (chrominance V) bits ______________________________________
Therefore, in the case of a PAL system, which representing the most burdensome case, may serve as a reference example, the actual total amount of memory required will be given by: EQU 1.835.008+835.584+4.976.640+4.976.640+(4.976.640*0.7407)=16.310.070 bits.
This calculation takes into account a 0.7407 optimization of the B-picture frame buffer.
A further optimization may includes carrying out the decompression of the B-picture without resorting to a storage step in the external RAM by carrying out an equivalent function internally in the integrated decoder device by a dedicated circuit block functionally placed upstream of the Display Unit, and commonly referred to as MACROBLOCK TO RASTER SCAN CONVERTER. This is implemented by employing an internal buffer for the B-picture made of high speed integrated memories, such as, for example, synchronous SDRAMs functioning at 100 Mhz and beyond.
Considering this further optimization, the video memory requirement is further reduced to: EQU 1.835.008+835.584+4.976.640+4.976.640=12.623.872 bits
because the internal B-buffer permits to convert the scanning of each 8*8 (pel) block, as defined in the MPEG-2 compressed data stream, into a line scanning of the field or frame picture required by the displaying process of the image on a TV screen. The European Patent Application No. 95830504.7, filed on Dec. 4, 1995, in the name of the present assignee, describes a method and a relative MPEG-2 decoder architecture according to which, through a recompression step of the I and P pictures, after the MPEG-2 decompression, according to an adaptive differential pulse code modulation (ADPCM) scheme, the memory requisite is remarkably reduced, to less than 8 Mbit, with a relatively small degradation of the image quality.